diff --git a/DynamoDbEncryption/dafny/DynamoDbEncryption/src/DDBSupport.dfy b/DynamoDbEncryption/dafny/DynamoDbEncryption/src/DDBSupport.dfy
index 6f3f93de9..6efe3e041 100644
--- a/DynamoDbEncryption/dafny/DynamoDbEncryption/src/DDBSupport.dfy
+++ b/DynamoDbEncryption/dafny/DynamoDbEncryption/src/DDBSupport.dfy
@@ -32,9 +32,34 @@ module DynamoDBSupport {
   import SET = AwsCryptographyDbEncryptionSdkStructuredEncryptionTypes
   import NN = DynamoDbNormalizeNumber
 
+  // Maximum length, in characters, of any single DynamoDB expression string
+  // (KeyConditionExpression, FilterExpression, ConditionExpression, etc.) that
+  // this library will parse. Matches DynamoDB's documented 4 KB server-side
+  // expression-string limit and bounds the work done by the client-side
+  // expression parser, preventing unbounded resource consumption from
+  // pathologically long inputs.
+  const MAX_EXPRESSION_LENGTH : nat := 4096
+
+  // Reject expression strings longer than MAX_EXPRESSION_LENGTH before they
+  // reach the parser. `name` is the human-readable field name used in the
+  // error message (e.g. "KeyConditionExpression").
+  function method ValidateExpressionLength(expr : Option<string>, name : string)
+    : Result<bool, Error>
+  {
+    if expr.None? then
+      Success(true)
+    else if |expr.value| <= MAX_EXPRESSION_LENGTH then
+      Success(true)
+    else
+      Failure(E(name + " exceeds maximum length of "
+                + String.Base10Int2String(MAX_EXPRESSION_LENGTH as int)
+                + " characters."))
+  }
+
   method GetNumberOfQueries(search : SearchableEncryptionInfo.BeaconVersion, query : DDB.QueryInput)
     returns (output : Result<PartitionCount, Error>)
   {
+    var _ :- ValidateExpressionLength(query.KeyConditionExpression, "KeyConditionExpression");
     var numberOfQueries :- Filter.GetNumQueries(
       search,
       query.KeyConditionExpression,
@@ -281,6 +306,8 @@ module DynamoDBSupport {
     returns (output : Result<(Option<DDB.ExpressionAttributeValueMap>, PartitionNumber), Error>)
     ensures output.Success? ==> output.value.1 < search.numPartitions
   {
+    var _ :- ValidateExpressionLength(keyExpr, "KeyConditionExpression");
+    var _ :- ValidateExpressionLength(filterExpr, "FilterExpression");
     if search.numPartitions <= 1 {
       :- Need(values.None? || PartitionName !in values.value, E("If no partitions are configured, do not specify " + PartitionName));
       return Success((values, 0));
@@ -309,6 +336,8 @@ module DynamoDBSupport {
     returns (output : Result<DDB.QueryInput, Error>)
     modifies if search.Some? then search.value.Modifies() else {}
   {
+    var _ :- ValidateExpressionLength(req.KeyConditionExpression, "KeyConditionExpression");
+    var _ :- ValidateExpressionLength(req.FilterExpression, "FilterExpression");
     if search.None? {
       var _ :- Filter.TestBeaconize(
         actions,
@@ -343,6 +372,8 @@ module DynamoDBSupport {
     ensures output.Success? ==> output.value.Items.Some?
     modifies if search.Some? then search.value.Modifies() else {}
   {
+    var _ :- ValidateExpressionLength(req.KeyConditionExpression, "KeyConditionExpression");
+    var _ :- ValidateExpressionLength(req.FilterExpression, "FilterExpression");
     if search.None? {
       var trimmedItems := Seq.Map(i => DoRemoveBeacons(i), resp.Items.value);
       return Success(resp.(Items := Some(trimmedItems)));
@@ -386,6 +417,7 @@ module DynamoDBSupport {
     returns (output : Result<DDB.ScanInput, Error>)
     modifies if search.Some? then search.value.Modifies() else {}
   {
+    var _ :- ValidateExpressionLength(req.FilterExpression, "FilterExpression");
     if search.None? {
       var _ :- Filter.TestBeaconize(
         actions,
@@ -414,6 +446,7 @@ module DynamoDBSupport {
     ensures ret.Success? ==> ret.value.Items.Some?
     modifies if search.Some? then search.value.Modifies() else {}
   {
+    var _ :- ValidateExpressionLength(req.FilterExpression, "FilterExpression");
     if search.None? {
       var trimmedItems := Seq.Map(i => DoRemoveBeacons(i), resp.Items.value);
       return Success(resp.(Items := Some(trimmedItems)));
diff --git a/DynamoDbEncryption/dafny/DynamoDbEncryption/test/DDBSupport.dfy b/DynamoDbEncryption/dafny/DynamoDbEncryption/test/DDBSupport.dfy
index cd169ba24..012d61aba 100644
--- a/DynamoDbEncryption/dafny/DynamoDbEncryption/test/DDBSupport.dfy
+++ b/DynamoDbEncryption/dafny/DynamoDbEncryption/test/DDBSupport.dfy
@@ -88,4 +88,58 @@ module TestDDBSupport {
     );
     result :- expect QueryInputForBeacons(Some(search), FullTableConfig.attributeActionsOnEncrypt, queryInput);
   }
+
+  // ValidateExpressionLength is the chokepoint that prevents
+  // pathologically long DynamoDB expression strings from reaching the
+  // recursive expression parser. Verify each branch of the helper.
+  method {:test} TestValidateExpressionLength() {
+    // None: nothing to validate.
+    expect ValidateExpressionLength(None, "FilterExpression") == Success(true);
+
+    // Empty string: trivially within the limit.
+    expect ValidateExpressionLength(Some(""), "FilterExpression") == Success(true);
+
+    // Short string: ordinary case.
+    expect ValidateExpressionLength(Some("a < :b"), "FilterExpression") == Success(true);
+
+    // Exactly at the limit (4096 characters): accepted.
+    var atLimit : string := seq(MAX_EXPRESSION_LENGTH, _ => ' ');
+    expect |atLimit| == MAX_EXPRESSION_LENGTH;
+    expect ValidateExpressionLength(Some(atLimit), "KeyConditionExpression") == Success(true);
+
+    // One over the limit (4097 characters): rejected with the expected
+    // error, which names the field that violated the constraint.
+    var overLimit : string := seq(MAX_EXPRESSION_LENGTH + 1, _ => ' ');
+    expect |overLimit| == MAX_EXPRESSION_LENGTH + 1;
+    expect ValidateExpressionLength(Some(overLimit), "KeyConditionExpression")
+        == Failure(E("KeyConditionExpression exceeds maximum length of 4096 characters."));
+    expect ValidateExpressionLength(Some(overLimit), "FilterExpression")
+        == Failure(E("FilterExpression exceeds maximum length of 4096 characters."));
+  }
+
+  // Confirm that the length check is wired into the public API surface:
+  // an over-long KeyConditionExpression must be rejected by
+  // QueryInputForBeacons before any expression parsing takes place.
+  // search := None is sufficient — the length check runs first,
+  // independent of whether searchable encryption is configured.
+  method {:test} TestQueryInputForBeaconsRejectsLongKeyExpression() {
+    var overLimit : string := seq(MAX_EXPRESSION_LENGTH + 1, _ => ' ');
+    var queryInput := DDB.QueryInput(
+      TableName := "SomeTable",
+      KeyConditionExpression := Some(overLimit)
+    );
+    var result := QueryInputForBeacons(None, map[], queryInput);
+    expect result == Failure(E("KeyConditionExpression exceeds maximum length of 4096 characters."));
+  }
+
+  // Same as above for FilterExpression on the scan path.
+  method {:test} TestScanInputForBeaconsRejectsLongFilterExpression() {
+    var overLimit : string := seq(MAX_EXPRESSION_LENGTH + 1, _ => ' ');
+    var scanInput := DDB.ScanInput(
+      TableName := "SomeTable",
+      FilterExpression := Some(overLimit)
+    );
+    var result := ScanInputForBeacons(None, map[], scanInput);
+    expect result == Failure(E("FilterExpression exceeds maximum length of 4096 characters."));
+  }
 }